Official BBC Arthur, Morgana and Dragon dress-up sets!
Just been on ebay looking if there was anything Merlin-related and I found these! (I so wish I were 8 years old - I want to be Arthur!!!)
http://cgi.ebay.co.uk/BBC-Merlin-Prince-Arthur-knight-fancy-dress-up-BNWT-7-8_W0QQitemZ390140077671QQcmdZViewItemQQptZUK_Clothes_Children_sFancyDress_RL?hash=item5ad6292667
http://cgi.ebay.co.uk/BBC-Merlin-Lady-Morgana-fancy-dress-up-costume-BNWT-7-8_W0QQitemZ390140104389QQcmdZViewItemQQptZUK_Clothes_Children_sFancyDress_RL?hash=item5ad6298ec5
http://cgi.ebay.co.uk/BBC-Merlin-Great-Dragon-fancy-dress-up-costume-BNWT-3-5_W0QQitemZ390140093870QQcmdZViewItemQQptZUK_Clothes_Children_sFancyDress_RL?hash=item5ad62965ae
But unfortunately no Merlin set :-(
ETA: The Ebayer selling the costumes told me: Strange as it seems, the BBC have brought out a range of Merlin costumes without actually including Merlin himself...
As soon as he becomes available, I hope to have him up on site - keep looking!